The electrochemical treatment of water to remove dissolved substances

Abstract:

The utilisation of electro-precipitation for the treatment of acid solutions on a laboratory scale to remove sulphate is explored. The electrolytic couple involved in the active process includes a sacrificial metal anode and inert cathode. Alternating or reversely polarised current is applied to the electrochemical reactor in order to curb the deleterious passivation of the electrodes. Mechanistically, oxidation of the anode is matched by the reduction of dissolved hydrogen ion to the H2 gas resulting in a pH increase in solution Sulphate removal is accomplished by precipitation of zinc-hydroxy-sulphate with unwanted zinc-hydroxide as by-product. Reduction of the sulphate content of the solution is accomplished using zinc plate anodes and cathodes. The final solution pH stabilises at a value of around 6.5 to 7 depending on the initial pH, with industrial water stabilising above 7. Stabilisation of solution pH reflects the attainment of kinetic equilibrium with respect to the rates of oxidation at the anode and diffusion of the oxidised products to the cathode where they precipitate as complex hydroxides. The investigation of the process is an empirical one. It employs statistical experimental designs in the execution of the experiments, and uses analytical and statistical methods to translate the process response into comprehensive terms. Through the manipulation of the process variables several process phenomena were identified. The untreated industrial wastewater proved to be an ideal electrolyte for the generation of the desired quantity of zinc ions. It was observed that temperature, concentration, competitive ions, time, acids, pH, current, agitation and different buffer solutions played an important role in the process. The dissertation describes the inter relationship and the effect of the above-mentioned variables and demonstrates the feasibility of the electrochemical removal of sulphate by generating zinc ions in industrial wastewater. This dissertation also looks at the reaction kinetics and energy relationships necessary for the process.